Oligosaccharides: Definition, Uses and Composition of These Molecules in Human Milk

They are molecules made up of the union of 2 to 10 cyclic monosaccharides.

Oligosaccharides are a carbohydrate. Polymers comprise three to ten monosaccharides or simple sugars. They were linked together primarily by O-glycosidic bonding through the condensation reaction between one anomeric carbon of one monosaccharide and the other.

They can also form N-glycosidic bonds under a certain atmosphere.

The minimum number of sugar reducing components is one molecule less than the total amount of simple sugars.

The reduction of sugar can be characterized from the hydroxyl group (-OH group) on the anomeric carbon.

Use of oligosaccharides

The number of oligosaccharide molecules can form polysaccharides through multiple bonds between the anomeric carbon at the end of one molecule and the hydroxyl groups on another oligosaccharide molecule.

Through the O-glycosidic bond and the N-glycosidic bond, the oligosaccharide can react with lipids and form lipopolysaccharides or saccharolipids.

N-linked oligosaccharides can also react with the side chain (s) of amino acid residues, particularly Asparagine of a protein, to form a Glycoprotein. Glycoprotein is not formed in the random part of proteins.

The glycoprotein is generally formed at a residue having an Asn-X-Ser or Asn-X-Thr sequence. However, not all of these residues will bind to the sugar molecule.

Usually they are linked due to nitrogen or oxygen bonds to compatible amino acids. Oligosaccharides are known to be found in glycolipids and glycoproteins.

Some of them are found from the decomposition of starch and cellulose, they are called cellodextrin and maltodextrin.

Chemical marking is one of the functions of oligosaccharides, this is because they have many variations and allow similarities.

For example, blood types are marked by oligosaccharides. Blood type ‘A’ has an oligosaccharide, blood type ‘B’ also has one. Both oligosaccharide markers are present in the ‘AB’ blood type, while the ‘O’ has neither.

The reason that blood must be typed before transfusion is because these oligosaccharides in the blood are different enough to be attacked by the body’s immune system.

Oligosaccharides can be identified as antigens detected by the immune system with incompatible blood as foreign pathogens.

If the transfusion occurs with incompatible blood types, clotting and major diseases will ultimately occur causing death.

However, type AB blood contains all possible possible oligosaccharide combinations (A or B) and since type O blood does not have attached markers, type AB blood transporters are generally called universal acceptors.

While type O blood carriers can only accept blood transfusions from other type O blood donors that do not contain any oligosaccharides present in red blood cells.


Galactooligosaccharides are synthesized through an enzymatic conversion of lactose. It is composed of chain units of the galactose group through consecutive transgalactosylation reactions, whose degree of polymerization varies from 2 to 8 monomeric units.

These new classes of prebiotics are known to play an important role in improving gut health by maintaining a balanced and beneficial gut microbiota. Today, many infant formula companies have galactooligosaccharides in their formula milk.

Human milk oligosaccharides

The oligosaccharides in human milk are complex glucans that can be found in human milk.

One of the most important factors in a baby’s diet is breast milk, which belongs to one of the most complex groups of oligosaccharides known as human milk oligosaccharides (HMO).

They are found in three, four, five or even six chain sugars. For example, some of the HMOs include raffinose, 2′-fucosyl-lactose, 3′-fucosyl-lactose, 3′-sialyl-lactose, 6′-sialyl-lactose, and Lacto-N-tetraose.

These HMOs differ in size, structure, and specific linkages. There are more than 150 different structures of Human Milk Oligosaccharides that have been identified so far. Also, these HMOs are different in structure, acidity, and functions.

The backbone of Human Milk Oligosaccharides is the disaccharide lactose, which is formed by the bond between galactose and glucose sugars.

The final structure of HMO depends on whether the main chain, lactose, is fucosylated or sialated, in beta or alpha configurations, or on a different carbon.

For example, 2′-fucosyl-lactose has a fucose group at the alpha-1-3 position of the glucose monosaccharide of lactose. Being sialylated means the addition of a sialic acid group and the formation of an acidic HMO.

HMO training

When the alpha-lactalbumin protein, also known as LALBA, is present, the enzyme beta-1,4-galactosyltransferase changes its function, thereby connecting galactose with glucose sugars, which form lactose.

The oligosaccharides in human milk are formed from lactose sugars, but the exact mechanism for this transformation is still unknown. HMOs are produced only when a woman is breastfeeding and are formed in the mammary glands.

HMO Investigations

Colostrum, which is the fluid secreted by women’s breasts for about several days in labor, is known to contain the highest amount of HMO, which varies by individual, but falls in the range of 20-30 g per liter. Colostrum contains more amounts of acidic HMOs compared to others.

Mature breast milk contains significantly less than about 10 g per liter.

Therefore, premature newborns are fed breast milk that contains a higher amount of HMO than a baby born after 37 weeks of gestation, since the milk has not had time to mature.

Although many medical professionals believe that breast milk is a healthier alternative to feeding newborns of HIV-positive mothers than formula, recent findings raise a problem.

Studies have suggested that the presence of a specific sugar, 3′-sialylactose, in breast milk increases the risk of an HIV-negative baby becoming infected with HIV from its HIV-positive mother.

However, of approximately 150 different HMOs, 3′-sialylactose has been the only one that has had a negative impact on the baby, while five others have had a positive effect.

Newborns who were fed breast milk that contained these five sugars lived longer than those who drank breast milk that did not contain the sugars.

With more research on the effects of HMOs in infants, the results could be applied to the research milk formula.

Since formula milk contains only small concentrations of complex oligosaccharides, unlike HMOs, studies can be conducted to determine if beneficial HMOs can be safely added to formula milk and have the same positive effect on newborns. born.

Additionally, women who produce breast milk that contains high levels of 3′-sialylactose may choose to formula feed their newborns, rather than breast milk. In general, breast milk is the most beneficial option for feeding a newborn, more than formula.

Studies clearly indicate that HMOs decrease the likelihood of pathogens attacking the respiratory, urogenital, and gastrointestinal tracts of newborns.

Studies can be done to determine if beneficial HMOs can be safely added to formula milk and have the same positive effect on newborns.

In general, breast milk is the most beneficial option for feeding a newborn, more than formula. Studies clearly indicate that HMOs decrease the likelihood of pathogens attacking the respiratory, urogenital, and gastrointestinal tracts of newborns.

HMO extraction 

HMOs can be successfully isolated from breast milk.

  1. First, the milk must be pasteurized to kill any bacteria.
  2. The milk is then subjected to centrifugation to separate the lipids from the aqueous phase.
  3. Subsequently, the proteins can be formed into granules and then removed.
  4. Finally, the sugars are left behind and can be separated by gel permeation chromatography, which separates the sugars according to their masses.